In drilling for natural gas or liquid petroleum, a drill string consisting of many lengths of threaded pipes screwed together and terminated by a drill bit head is used to bore through rock and soil. The drill bit head has a larger diameter than the pipes forming the drill string above it. The upper end of the drill string is rotated to transmit a rotary boring action to the drill bit head.
In a typical drilling operation, a specially formulated mud is introduced into an opening in an upper drill pipe. The mud flows downward through the hollow interior of the pipes in the drill string and out through small holes or jets in the drill bit head. Since the drill bit head has a larger diameter than the drill string above it, an elongated annular space is created during the drilling process which permits the mud to flow upwards to the surface. The purpose of the mud is to cool the drill bit, carry cuttings to the surface, and provide a downward hydrostatic pressure which counteracts pressure which might be encountered in subsurface gas pockets.
In normal oil well drilling operations, it is not uncommon to encounter subsurface gas pockets whose pressure is much greater than could be resisted by the hydrostatic pressure of the elongated annular column of drilling mud. To prevent the explosive and potentially dangerous and expensive release as gas and/or liquid under pressure upwards out through the drill hole, blowout preventers are used. Blowout preventers are mounted in a pipe casing surrounding a drill hole, near the upper end of the hole.
Typical blowout preventers have a resilient sealing means which can be caused to compressively contact the outer circumferential surfaces of various diameter drill string components, preventing pressurized liquid or gas in subterranean pockets from blowing out material along the drill string. Usually, the resilient sealing means of a blowout preventer is so designed as to permit abutting contact of a plurality of individual sealing segments, when all elements of a drill string are removed from the casing. This permits complete shutoff of the well, even with all drill string elements removed. Most oil well blowout preventers are remotely actuateable, as by a hydraulic pressure source near the drill hole opening having pressure lines running down to the blowout preventer.
Ram blowout preventers (BOP's) utilize a pair of opposed semi-circular disc-shaped sealing elements driven radially inwards into peripheral sealing contact with tubular pipe or other drill string components extending through the bore of the BOP. Each of the semi-circular ram sealing elements has a flat diametrical face in which a coaxial, semi-circular groove is formed. The grooves are adapted to conformally engage opposite halves of the cylindrical surface of a tubular drill string component.
Usually, resilient elements are incorporated into the ram sealing elements which allow the notched faces of the two sealing elements to form a tight seal against the periphery of a tubular drill string component within the bore of the BOP. Providing resilient elements allows a pressure-tight seal to be made around the periphery of tubular drill string components having a slight variation in outer diameter. The seal must be effective against well-hole pressures as high as 15,000 psi.
Variable bore ram rubbers or sealing elements are used in the drilling and workover of deep oil and gas wells when the drill string is made up of pipes of different sizes, forming what is referred to as a tapered string. Ram rubbers of variable bore design are adapted to effect pressure-tight seals against the peripheral surface of pipes of various diameters.
Variable bore rams in current use employ a special variable bore ram block which is structurally different from ram blocks intended for use with fixed diameter drill string components.
Variable bore ram blocks have a deeper cavity or, "rubber pocket" which is necessary to provide sufficient rubber to effect a positive seal with various diameter pipes. Because of the use of special ram blocks, variable bore ram assemblies are considerably more expensive than standard, fixed bore ram assemblies. Consequently, most small drilling operators do not use variable bore ram assemblies, and their use is limited even among larger operators. However, more extensive use of variable bore rams would be desirable, since their use can provide substantial savings in operating time, by eliminating the requirement of halting movement of drill pipe to change to a different size, fixed-bore ram assembly for each size of pipe in a drill string.
In addition to the operating time savings afforded by the use of a variable bore ram, there are safety advantages. Thus, a variable bore ram assembly may be actuated to form an effective blowout seal on piping of any diameter in the bore of the ram assembly.
On the other hand, with a fixed bore ram blowout preventer, if a drill string needs to be pulled quickly due to some emergency, the fixed bore ram assembly is unable to effect a seal when piping of smaller diameter than that which it is designed to seal is present in the bore of the ram assembly.
From the foregoing discussion, it should be apparent that it would be desirable to have a variable bore ram rubber which would fit in a standard, fixed bore pipe ram block. Also, it would be desirable to have variable bore ram rubbers which would overcome certain limitations inherent in existing variable bore ram rubbers, as will now be described.
Existing variable bore ram rubbers typically include a uniform thickness sealing element generally shaped in plan-view cross-section somewhat like a symmetrical, semi-circular arch. Additionally, some sealing elements have straight, coplanar legs joining the opposite ends of the arch, and extending laterally outwards therefrom. One such sealing element is installed in each of two opposed, semi-circular ram blocks, the flat bases of the legs of opposing sealing elements abutting each other to form a pressure-tight seal, and the concave semi-circular surfaces of opposed arches sealing against opposite cylindrical sides of a tubular drill string component passing through the bore of the ram sealing assembly, the bore being defined by the union of the two semi-circular cross-section arches.
To provide the necessary resilience to form an effective seal, existing sealing elements usually include a molded rubber matrix. Metal segments or inserts are molded into the matrix to provide required strength and rigidity to the sealing element. Typically, the metal segments or inserts are coextensive with the thickness dimension of the sealing element.
Resilient sealing elements of ram blowout preventers require rugged metal inserts to be interspersed with the resilient rubber matrix to add strength to the sealing elements. Such strength is required because the required sealing forces on a five-inch diameter pipe, for example, can be as high as 500,000 lbs. Also, the sealing element is sometimes required to grip a long string of drill pipe to prevent it from falling down into the well hole. The weight of a string of 5-inch drill pipes can approach the tensile strength of pipe, i.e., 600,000 lbs.
Most prior art variable bore ram sealing elements use a large number of metal inserts or segments, typically 10 to several dozen. The large number of segments is used to permit the sealing elements to conform to various pipe sizes, usually in the range diameter between 31/2 inches and 5 inches.
Prior art variable bore ram sealing elements utilizing a large number of metal segments include Nelson, U.S. Pat. No. 4,332,367, July 1, 1982, which discloses the use of non-overlapping segments, and Le Rouax U.S. Pat. No. 3,915,426, Oct. 28, 1975, which discloses the use of overlapping segments.
The use of a substantial number of metal segments in existing variable bore ram sealing elements has a number of disadvantages. One such disadvantage results from the fact that every interface between segments has rubber which can extrude between the segments. Extruded rubber is pinched and cut off each time the seal is compressed into a closed position, decreasing the life of the seal.
Another disadvantage of using a large number of metal segments in a variable bore ram sealing element is that the small size of the segments makes their movement towards one another during compressive sealing somewhat unpredictable, owing to the fact that some segments will stick and some will move more readily than others. This causes gaps in sealing effectiveness to occur, especially at the corners of the sealing elements, i.e., the junctions between the straight legs and arch of a sealing element half. Also, it would be desirable for the sealing element to have greater strength at the corners, where stresses are greater during compressive sealing. This strengthening is not feasible with existing sealing elements which employ a large number of metal segments of generally uniform size and shape.
With the aforementioned limitations of prior existing variable bore ram rubber seals in mind, the present inventors conceived of an improved variable bore ram rubber. The improved sealing element, disclosed in U.S. Pat. No. 4,930,745, June 5, 1990 Granger et al., Variable Bore Ram Rubber, utilizes a sparse number of slidably interleaved inserts imbedded in a rubber matrix to provide a sealing apparatus which is highly effective for use with drill string components lying within a broad range of diameters, 21/2 inches to 5 inches, for example. The present invention was conceived of to provide a simplified variable bore ram rubber which is useable over a somewhat reduced diameter range and pressure range than the aforementioned improved sealing element.